This invention relates to the formation of a plasma using microwave energy, and in particular to an efficient, compact, high-output plasma source for use with deposition systems. Such deposition systems may be used in fabricating semiconductor wafers, for example.
A plasma is a partially ionized gas containing positive and negative ions and radicals. These ions and radicals may be beneficial in many types of operations used in processing semiconductor wafers. Examples of such operations include plasma-enhanced chemical vapor deposition (PECVD), sputtering deposition, sputter etching, reactive-ion etching (ion-assisted sputter etching), and plasma etching (ionic-chemical) or cleaning processes.
One way a plasma can be created is to apply an electric field across a gas. Under the proper conditions, free electrons (generated by a passing cosmic ray, spark discharge, or ultraviolet light source, for example) are accelerated to sufficient energy so that an inelastic collision between the electron and a gas molecule results in the ionization of the molecule. A direct current (DC) field may be used to create a plasma, but high-frequency fields, such as radio frequency (RF) or microwave frequency (MW) are preferred to generate the desired plasma species as high-frequency fields produce a greater number of inelastic collisions. Some plasmas may form a glow discharge.
A glow discharge is a condition where there are a sufficient number of free electrons available to sustain a glowing plasma region. A self-sustaining number of free electrons may form when the energy transferred in an inelastic collision between an electron and a gas molecule (including single-atom gases) or ion is greater than the ionization potential for that gas molecule or ion. The collision may create a second free electron, and both the original and secondary electron may then be accelerated to an energy sufficient to create two new ionizing collisions, thus cascading the number of free electrons available to maintain a stable plasma. In this state, inelastic collisions between electrons and gas molecules that are insufficient to liberate a free electron may briefly excite an electron to a higher orbital state. As these excited electrons collapse to their ground state, they may release photons, often in the visible spectrum. This causes the plasma to glow, and gives the glow discharge its name.
Many processes create a plasma, often in the form of a glow discharge, at the location of the desired operation, or process zone. PECVD, for example, typically uses such an in situ plasma because the physical movement, as well as the chemical activity, of the plasma species is important to the process. Some other processes, such as some plasma etching or cleaning processes, do not require the plasma to be generated at the process zone. The desired plasma species created in the glow discharge may have a sufficient lifetime before recombining into neutral species such that the plasma species may be used in a location remote from the plasma activation (glow discharge) zone.
FIG. 1 shows a deposition system that utilizes one type of a prior art remote plasma source. In this system, a microwave source 100 is connected to a waveguide 110 that irradiates process gas in the discharge tube 120 to form a glow discharge 111. The discharge tube conveys ions and radicals resulting from the glow discharge 111 to the processing chamber 130 through the applicator tube 121. This may be an inefficient way to create a plasma for at least three reasons. First, a waveguide typically transmits only certain modes of microwave fields, reducing the power from the microwave source, which may generate power in many different modes, to the plasma interaction (glow discharge) zone. Second, many ions may recombine into molecules as they travel down the applicator tube to the process zone, reducing the number and/or concentration of ions available for the process. Finally, the potential volume available for interaction between the plasma and the microwaves may be limited by the cross-section of the waveguide and the crosssection of the discharge tube. This may limit the generation of ions and radicals at a given pressure and flow within the discharge tube because the volume of the plasma activation zone (interaction volume) is small compared to the total volume of the applicator tube; for example, some designs use a two-inch lengthwise section of a tube having a one-inch diameter for the plasma activation zone. Because the interaction volume is so small, high-power-density, relatively expensive, DC microwave power supplies are typically used in order to obtain sufficiently high microwave coupling.
However, it is more expensive to operate such high-power-density supplies than 100-W to 700-W magnetrons, for example. In addition, remote plasma systems that transmit high-power microwave energy through a waveguide to a small interaction zone concentrate the heat generated by the plasma in a fairly small volume. Therefore, conventional remote plasma systems often use a cooling system to keep the applicator tube from overheating. Typically, these are liquid-cooled systems, which add initial expense and maintenance costs. Also, liquid-cooled systems often suffer from leakage problems. Such leakage may lead to corrosion of the equipment, which may cause a degradation in the quality of the processed substrates.
Another type of problem related to the type of remote plasma system shown in FIG. 1 is that it is complicated and bulky. This may affect the time and effort required to disconnect the remote plasma system for chamber maintenance, especially if a cooling system is involved. Additionally, if this microwave system is improperly reconnected afterwards, microwave energy may radiate into the surrounding room, posing a safety threat to personnel and interfering with electronic devices.
FIG. 2 shows another type of remote plasma source used with wafer processing chambers, sharing some of the disadvantages of the system shown in FIG. 1. In the system shown in FIG. 2, a microwave antenna, or launcher 201, couples energy from a microwave source 200 into a resonator cavity 240. The microwave source 200 may be a 2.45-GHz magnetron, for example. While a greater volume of gas, compared to the system of FIG. 1, may be irradiated with microwave energy in the configuration shown in FIG. 2, the volume of the discharge tube 220 within the resonator cavity 240 is substantially less than the total volume of the discharge tube 220 and the applicator tube 221. As described above, recombination of some of the plasma ions and radicals desired for processing may occur in the applicator tube 221, reducing the concentration of ions radicals in the plasma that reach the processing chamber 230.
Another problem with some plasma systems, such as those shown in FIGS. 1 and 2, arises from the variable absorption of microwave energy by the plasma as the plasma density changes. When microwave energy is first transmitted to the gas in the applicator tube it may accelerate electrons to sufficient energy so that the electrons collide with gas molecules, as discussed above. These collisions may create additional free electrons and other ionic species. At the onset of irradiation, the gas may be nearly electrically nonconductive. As the ion and free electron concentration increases, the plasma becomes more and more conductive, increasing in conductivity by up to two orders of magnitude, or more. The plasma may become so conductive that it reflects a significant portion of the microwave energy that impinges on it. At some point, the plasma will reach a critical density, where it will not absorb any additional energy. This is called the critical density (N.sub.C), and is about 7.times.10.sup.10 ions/cmn.sup.-3 at 2.45 GHz. At this point, the plasma may become unstable and flicker, and the high amount of energy reflected from the plasma back to the microwave source may damage the source. The typical response is to operate the plasma system well below N.sub.C. However, this means that the ion density in the plasma is well below the maximum possible concentration. To prevent damage to the microwave source, matching networks, such as mechanically tunable stubs, have been used to improve the power transfer efficiency between the source and the load (plasma) in some systems. However, because the impedance (conductivity) of the plasma may vary over such a wide range, it may be difficult to obtain a good match over the entire range of plasma densities and operating conditions.
From the above, it can be seen that it is desirable to have a compact remote microwave plasma system that efficiently produces a high concentration of ions for cleaning of chemical vapor deposition (CVD) apparatus and other apparatus. Such a system should provide a reasonably constant ion density from a stable plasma and should not expose the processing chamber to the potentially harmful effects of a glow discharge. It is further desirable that the remote plasma system be simple, preferably small enough to fit on the lid of a processing chamber.